Mastocytosis is a disease of disordered mast cell proliferation which affects all ages. Clinical presentation and prognosis depend on the category of disease, which ranges between a benign and self-limited skin disease to more aggressive forms associated with hematologic disorders. The most common sites of mast cell accumulation observed in patients with mastocytosis are skin and bone marrow. Skin lesions may eventually fade in some patients with longstanding disease. The pathologic basis of the mast cell collections in skin is not known; however it does not appear to be due to increased levels of mast cell growth factors in skin. In some cases, mastocytosis has an aggressive and ultimately fatal course. Thus, our efforts are directed to improving diagnosis and treatment; and to clarifying the etiology of this disease. Studies on c-kit and the relevance of activating mutations which we first identified (Asp816Val; Asp816Tyr) in patients with mastocytosis are continuing. Polymorphisms in other genes regulating mast cell growth and differentiation may influence the pathologic effects of c-kit mutations. Thus, a polymorphism in the IL4 receptor alpha chain has been identified to be associated with milder forms of mastocytosis. A successful approach to treatment for aggressive disease remains elusive. Currently available tyrosine kinase and Kit inhibitors do not appear to have a significant inhibitory effect on mutated Kit associated with mastocytosis. The efficacy of bone marrow transplantation in patients with severe bone marrow disease is currently being investigated.